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PDF AN9622.2 Data sheet ( Hoja de datos )
| Número de pieza | AN9622.2 | |
| Descripción | Using HFA3724EVAL Evaluation Board | |
| Fabricantes | Intersil | |
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Using the PRISM® HFA3724EVAL
TM Evaluation Board
Application Note
March 2000
AN9622.2
Author: Raphael L. Matarazzo
Introduction
The HFA3724 is a highly integrated IF
strip and baseband converter for half
duplex wireless data applications. It
features all the necessary blocks for
baseband demodulation and
modulation of “I” and “Q” quadrature multiplexing signals. It
modulation/demodulation. A buffered, divide by 2, LO single
ended 50Ω selectable output is provided for convenience of
PLL designs. The receive down converter mixers “I” and “Q”
outputs have a frequency response up to 30MHz for
Baseband signals and the transmit mixers outputs are
summed and amplified to a single ended open collector
output with frequency response up to 400MHz.
targets applications using all phase shift types of modulation
Multiplexed or half duplex baseband 5th order Butterworth
(PSK) due to its limiting receiving front end. Four fully
independent blocks adds flexibility for numerous applications
mcovering a wide range of IF frequencies. Figure 1 depicts the
simplified block diagram of the HFA3724.
oThe HFA3724 has a two stage integrated limiting IF amplifier
.cwith frequency response to 400MHz. These amplifiers
exhibit a -84dBm, -3dB cascaded limiting sensitivity with a
built in Receive Signal Strength Indicator (RSSI) covering
U60dB of dynamic range with excellent linearity. An up
conversion and down conversion pair of quadrature doubly
t4balanced mixers are available for “I” and “Q” baseband IF
processing. These converters are driven by an internal
equadrature LO generator which exhibits a broadband
response with excellent quadrature properties. For
ebroadband operation, the Local Oscillator frequency input is
hrequired to be twice the desired frequency for
low pass filters are also included in the design. The “I” and
“Q” filters address applications requiring low pass and
antialiasing filtering for external baseband threshold
comparison or analog to digital conversion in the receive
channel. During transmission, the filters are used for pulse
shaping and control of spectral mask.
Four filter bandwidths are programmable, (2.2MHz, 4.4MHz,
8.8MHz and 17.6MHz) via a two bit digital or hardwired
control interface. These cut off frequencies are selected and
can be fine tuned for optimization of spectrum output
responses.
Ordering Information
PART NUMBER
HFA3724EVAL
PKG. NO.
Evaluation Kit
ataSLIM1_IN
RSSI1
.DRSSI2
IF
MOD_LO_IN
wMOD_LO_OUT
wLO_GND
mMOD_TX_IF_OUT
w .co2V
REF
IF
÷ 2 0o/90o
I
M
U
X
Q
∑
M
U
X
LPF_RXI_OUT
LPF_RXQ _OUT
LPF_TXI_IN
LPF_TXQ_IN
Sheet4UFIGURE 1. BLOCK DIAGRAM
www.Data4-1
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1 page  
Application Note 9622
All LPF settings can be tested by varying the 2XLO
frequency generator accordingly. The user need to use
caution for very low frequency “I” and “Q” signals because of
the use of coupling capacitors in the evaluation board from
the down converter mixers to the LPF input and from the
LPF output to the on board 2kΩ resistor load. These
components create a high pass filter action with a 3dB point
of approximately 15kHz for 0.01µF decoupling capacitors.
An indirect method of the 3dB LPF filter evaluation is by
setting a reference signal to the vector voltmeter at 1MHz for
example (562MHz 2XLO as in our example). This level
corresponds to a 0dB when this reference signal is saved in
the voltmeter. Increasing the 2XLO frequency in fine steps will
start moving the reference down until the vector voltmeter
reads -3dB. This frequency setting (2XLO) is divided by 2 and
subtracted from the IF frequency of 280MHz (for example) to
find the 3dB point of the LPF filter. Note: Some vector
voltmeters cannot scan or keep probe amplitude calibration
for different frequencies and errors can occur.
As all LPF filter settings (2.2, 4.4, 8.8 and 17.6MHz) are slightly
different from each other, it is suggested to calibrate a new
reference 1MHz signal for each of the desired settings.
Another method of low pass characteristics evaluation is by
using a network analyzer. Although a little more complicated,
this method is useful to evaluate the overall group delay
characteristics of the Demodulator at all LPF settings and
will be explained later in this section.
RSSI TESTS
The RSSI characteristics of the HFA3724 can be evaluated
by monitoring the voltage level at the RSSI output pin
available in the evaluation board. This pin is located and
labelled in a flat cable connector used to interface with the
Intersil HSP3824 Base Band Processor.
The CW IF input level is varied from -90dBm to 0dBm for
example, in steps, and the corresponding voltages in the
RSSI output can be monitored and recorded. A typical RSSI
output curve is depicted in Figure 4.
The RSSI output pin is sensitive to long wire loops as it can
generate signal feedback to the IF input. This situation can
lead to possible oscillation or increase in spurs from the IF
amplifier outputs. Use caution when using long lead voltmeter
probes at this pin. When performing the limiter output
spectrum check earlier in this section, the user can make an
assessment of the sensitivity of the RSSI line by placing and
removing lead voltmeter wires at the RSSI test point.
The HFA3724 RSSI outputs are a sum of the two current
outputs of each of the limiters. These current sources have
been summed together through two on chip 6kΩ resistor in
parallel. Zero ohm resistors have been added to feed the
current outputs to the respective resistor pins (pins 60 and 80).
100mV/DIV.
1.5V
1.0V
0.5V
-100
VCC = 3V
-80 -60 -40 -20
INPUT POWER (dBm INTO 50Ω)
0
FIGURE 4. TYPICAL RSSI RESPONSE
The user can modify the voltage range when required, by
having the two current outputs summed through a different
resistor value.
Demodulator Group Delay Characteristics
Group delay and filter passband characteristics can be
evaluated by a network analyzer by using a set up as in
Figure 6.
The network analyzer generator sweep baseband signal is
upconverted to an IF signal by a doubly balanced mixer. The IF
signal is a double side band suppressed carrier signal contrary
to a CW signal as in the previous sensitivity tests paragraphs.
This DSB signal can be conveniently used for the “I” and “Q”
evaluation individually when a proper phase shift technique is
used. The doubly balanced mixer must have a reasonably RF
input frequency response down to a few hundreds of kilohertz
and good passband characteristics across a 40MHz bandwidth
which is very common for broad band diode mixers. The
network analyzer generator output is split between its reference
input “R” and the RF port of the mixer. A 280MHz signal
generator is used for the LO input of the mixer and also as a
reference LO for the HFA3724. The 2XLO signal for the LO_IN
input of the device is generated by an off the shelf frequency
doubler (a second generator locked to the 280MHz generator is
not suggested unless they have very good phase lock
characteristics). The output of the frequency doubler is routed
through a line stretcher for phase shift control.
Mathematical manipulation of a double side band input
signal and the response of a quadrature demodulator
suggests a phase relationship between the LO and the
input signal such that one and only one of the outputs “I”
and “Q” signals is present. The line stretcher is used to
cancel the channel not being evaluated.
The outputs “I” and “Q” result from the following trigonometric
equations:
I = 1/2cos(wmt-Θ) + 1/2cos(wmt+Θ) and
Q = 1/2sin(wmt-Θ) - 1/2sin(wmt+Θ)
4-5
5 Page 
Application Note 9622
network analyzer generator. This TTL signal will sweep as
desired by the user. Only one of the channels must be
evaluated at the time due to the summing of the upconverted
“I” and “Q” signals. It is important to note that the input of a
baseband signal of either “I” or “Q” by itself will generate a
dual side band signal at the HFA3724 output. This dual side
band signal is down converted back to the baseband by an
external broad band doubly balanced mixer. The main
restriction for the external mixer is its IF output frequency
response which should cover from 500kHz and up. The
external mixer receives its LO input from a 280MHz CW signal
generator. The HFA3724 output signal is mixed down by this
external mixer yielding a baseband signal to be monitored by
the network analyzer. The HFA3724 2XLO is also generated
by a doubler from the same 280MHz CW generator. Again,
proper phase shift between the external 2XLO signal and the
external mixer LO is required for proper operation of the down
conversion process from a dual side band signal. A line
stretcher is used and adjusted after the frequency doubler for
maximum reading of the external mixer IF output.
Other Test Setups
Although most of the test set ups described earlier can
evaluate the performance of the HFA3724, the reader is
welcome to evaluate the device in a more system oriented
manner with the common test procedures that follow:
Figure 15 depicts a common QPSK demodulation test set up
for the HFA3724 using a Vector Signal Generator at a
280MHz carrier frequency, modulated by a spread digital
sequence (Again, the HSP3824 evaluation board is more than
suitable for this data generation). Coherence between the
Carrier generator and the HFA3724 is done by the use of a
line stretcher for the 2XLO signal and by tying the reference
10MHz signals often offered by these equipments. A
modulation analyzer can be used for displaying the
constellation characteristics and eye diagram results of the
demodulation process. Figure 12 shows a typical Baseband
QPSK constellation (Vector States) as the input to the Vector
modulator used in Figure 15.
Figure 13A and Figure 13B depicts the constellation and eye
diagram of the device for a 11MHz symbol rate with the
LPF8.8 filter setting at 8MHz. The Carrier generator output
signal has been set to -60dBm.
Another method, more realistic, which accounts for
prefiltering before transmission (spectrum shaping) is using
a second HFA3724 evaluation board as the generating
signal carrier (The reader may ask “Why not use the same
board in analog loop back mode?” As the LPF filters are
multiplexed between Transmit and Receive and the ref LO is
the same, this application is not possible).
Figure 16A shows a Modulation/Demodulation test setup.
Figure 16B depicts the same set up by using the HSP3824
evaluation board as the baseband generator. Again,
synchronization or coherence is needed for proper evaluation.
TIMEBASE = 10.0ms/DIV.
VECTOR
Q vs I
TIMEBASE = 10.0ms/DIV.
X-GAIN = 300.0mV/DIV.
Y-GAIN = 300.0mV/DIV.
FIGURE 12. TYPICAL 11MHz QPSK BASEBAND
CONSTELLATION
VECTOR
Q vs I
TIMEBASE = 10.0ms/DIV.
X-GAIN = 100.0mV/DIV.
Y-GAIN = 100.0mV/DIV.
FIGURE 13A. HFA3724 DEMODULATOR OUTPUT
CONSTELLATION
I vs Q vs TIME
TIMEBASE = 10.0ms/DIV.
X-GAIN = 200.0mV/DIV.
Y-GAIN = 200.0mV/DIV.
FIGURE 13B. HFA3724 DEMODULATOR EYE PATTERN
Contrary to a vector generator, which can have its output level
well controlled, it is necessary to provide an attenuation path
(attenuator) from the output of the HFA3724 transmitting board
to the IF input of the receiving board for sensitivity level
evaluation. (The output signal is approximately -7dBm.)
4-11
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| PDF Descargar | [ Datasheet AN9622.2.PDF ] | |
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